1. Field of the Invention
The present invention relates to an endoscope objective lens unit arranged at a distal end portion of an endoscope and an endoscope including the endoscope objective lens unit.
2. Description of the Related Art
In a medical field, endoscopes are used for, e.g., treatment/diagnosis of sites that are difficult to observe from outside of bodies of patients. There is a large demand for a further diameter reduction in endoscopes with small diameters, represented by nasal endoscopes. Reduction in diameter of endoscopes largely depends on development of small-sized image pickup devices such as CCDs, and pixel pitches of image pickup devices are reduced year by year. Accompanied by such reduction, there is a need for endoscope objective lens units (hereinafter also referred to as “lens units”) to provide satisfactory performance while achieving size reduction, and various types of configurations have been developed.
For example, in Japanese Patent Application Laid-Open Publication No. 6-308381, the present applicant discloses an endoscope objective lens including a front lens group and a rear lens group with a diaphragm interposed therebetween, in which the front lens group includes a first group of negative lenses and a second group having a positive refractive power in this order from an object side, the second group has a shape including a surface having a small curvature radius provided on an image side, the rear lens group includes a positive single lens and a cemented lens of a positive lens and a negative lens, and the endoscope objective lens satisfies a predetermined condition.
Meanwhile, in Japanese Patent Application Laid-Open Publication No. 2006-51132, the present applicant also discloses a lens unit including a lens that includes a high refractive index material that exhibits excellent sterilization durability.